This invention relates to methods for shifting twin-clutch transmissions having two transmission input shafts and a transmission output shaft, each input shaft having a friction clutch in which initially one of the two clutches transmits an engine torque in a state of static friction and the other clutch is open.
The invention also relates to a twin-clutch transmission having two transmission input shafts and a transmission output shaft for operation in accordance with a gear-shifting method according to the invention.
Manual transmissions used in motor vehicles are simple and robust but have the disadvantage that there is an interruption in the drive connection during gear shifting. In passenger vehicles, conventional manual transmissions are generally provided with single-cone synchronizers of the Borg-Warner type.
Although conventional automatic transmissions avoid an interruption in the drive connection during gear shifting, their efficiency is poor, among other reasons because of the need for a hydrodynamic converter for starting. Furthermore they are very heavy.
Attempts have therefore been made to shift conventional transmissions automatically and, in particular, under load.
Among the power-shift transmissions are twin-clutch transmissions of the PDK type i.e., the Porsche Twin-Clutch Transmission. This is a transmission with two transmission input shafts which act on a common transmission output shaft, and, as a rule, the two transmission input shafts are arranged coaxially with respect to each other. The gear wheels for the even-numbered transmission gears are generally located on one transmission input shaft and the gear wheels for the odd-numbered transmission gears are generally located on the other transmission input shaft. Each of the two transmission input shafts can be connected to the internal combustion engine with either sliding friction or static friction by a dedicated independent clutch.
In this way, it is possible to perform gear changes without an interruption in the drive connection. Theoretically, this makes it possible to provide automatic transmissions which have a high level of efficiency. However, this requires the use of conventional synchronizer rings.
In the design of automatically actuated twin-clutch transmissions it has been found that controlling and regulating such transmissions is complicated and that the expected vehicle occupant comfort cannot be achieved. Experience shows that gear shifting transmissions in which gear shifting is automated using the conventional locking-type synchronizer rings are associated with considerable unexpected problems. In particular, the forces and speeds of the hydraulic shifting can lead to failure of the synchronizer.
Efforts have therefore been made to operate hydraulic actuating systems with modulated pressures. The expenditure required for this purpose results in excessive costs which are unacceptable for the mass production of motor vehicles.
Other power-shift transmissions having a single start-up clutch employ an auxiliary clutch which must be provided in the highest transmission gear to enable a filler torque to be produced when changing up. This filler torque helps to minimize the interruption in the drive connection during a gear change.
Finally, there have been central synchronizers for the entire transmission for the purpose of avoiding the problems mentioned above when using the conventional locking-type synchronizer rings with hydraulic actuation. With central synchronization, a central synchronizing unit is arranged in such a way that a transmission output shaft, for example, can be braked or driven. For this purpose, additional mechanical, hydraulic or electric drives are necessary and this leads to excessive costs and weight in normal passenger vehicle construction.
Finally, engine control concepts have been developed in which control of the speed of the transmission input shaft has been carried out by appropriate control of the operation of the internal combustion engine. Because of the high masses involved and the resulting high inertias, such a method is not advantageous. Furthermore, the driver may have a subjective impression of helplessness. The increase in engine speed which naturally occurs when shifting down-on a downhill slope, for example, may frighten the inexperienced driver.
A further negative effect of conventional synchronizing operations is that torques are required for accelerating or braking the transmission input shaft, and these act on the transmission output shaft and hence on the vehicle. If the synchronizing operation is to be performed rapidly, as required for passenger vehicles, for example in less than one half-second, very strong vehicle reactions can occur since, when synchronization is achieved, the synchronizing torques collapse abruptly and the resulting large change in torque is experienced by the vehicle occupants as a powerful jerk. In order to avoid such discomfort, the modulated pressures mentioned above have been employed on automated manual transmissions having hydraulic actuating elements.
In conventional automatic transmissions, electronically synchronized transmissions, or power-shift transmissions, the disengagement of gears during shifting is frequently effected by a spring mechanism. The gears are normally held in their engaged position by an undercut in the respective dog toothing. If the torque transmitted in the transmission gear falls below a selected value, the spring force can push the shift dog out, disengaging the gear. The disadvantage with this method is that the torque at which the gear disengages is not zero, i.e. there is a torque jump, producing the above-described discomfort. It is also possible in some cases for the transmission mechanism to be distorted. Moreover, shifting up in overrun is fundamentally impossible for the power-shift transmissions mentioned above without performing an engine management operation, i.e. the negative engine torque must be increased to at least zero. This results in problems for the driver which are mentioned above since the driver may not understand various reactions of the engine and, in cases of doubt, may have a growing subjective sense of danger.